1. Field of the Invention
This invention relates to an electrochromic display device. More particularly, it relates to an electrochromic display device which utilizes color change of an electrochromic compound caused by its reaction with a counter electrode material through an electrolyte.
2. Description of the Prior Arts
A typical electrochromic display device comprises a display electrode comprising a transparent substrate, a transparent electrode formed on an inner surface of the transparent substrate and an electrochromic material layer for display formed on the transparent electrode: a counter electrode comprising a substrate, a conductive layer formed on an inner surface of the substrate and a counter electrode material; and a spacer provided between the display electrode and the counter electrode with their inner surfaces facing each other to define, between them, an electrolyte chamber which is filled with an electrolyte.
Such electrochromic display device utilizes coloring of the electrochromic material by applying voltage between the the display electrode and the counter electrode through the electrolyte.
Generally, tungsten oxide (WO.sub.3) is used as an electrochromic material, and a lithium salt is used as an electrolyte. In such combination of the electrochromic material and the electrolyte, tungsten oxide turns blue according to the following reaction: ##EQU1##
In order to increase an amount of injected charge so as to improve response characteristics of the electrochromic display device comprising tungsten oxide as an electrochromic material and a solution of a lithium salt in a non-aqueous solution as an electrolyte, water is added to the electrolyte, and an activated carbon material such as activated carbon fiber is used as a counter electrode material. Water is dissociated to liberate H.sup.+ which acts in the same manner as Li.sup.+ in the electrode
Although the increase of the amount of water in the electrolyte achieves the desired response characteristics of the electrochromic display device which utilizes the activated carbon material as the counter electrode, injected charge gradually decreases during storage to a greater extent and it is difficult to maintain stably the original response characteristics for a long time.